An optical time domain reflectometer (OTDR) is an instrument used to measure attenuation in a fiber optic cable. The OTDR allows the fiber attenuation to be correlated to the length or distance along the fiber cable. The OTDR has the advantage that it requires neither cutting the fiber nor the need to access both ends of the fiber.
In operation, a pulse of light is launched into the fiber in the forward direction by using either a directional coupler or a system of external lenses and a beam splitter. The waveform of the return light pulse is detected by a photo-detector and processed with a boxcar integrator (to improve the signal-to-noise ratio, i.e. SNR). The return waveform comprises three segments or components: (1) an initial pulse; (2) a long tail caused by back-scattering; and (3) pulses caused by discrete reflections. The three segments or components comprise optical features. The initial pulse results from any lack of directionality in the input coupling mechanism. The long tail caused by scattering results from Rayleigh scattering that occurs as the input light pulse propagates down the fiber and the reflection or pulses are caused by mismatched connectors or changes in the dielectric interface for example.
Rayleigh scattering is the major factor contributing to attenuation of an optical signal propagating through an optical fiber. Rayleigh scattering is due to microscopic fluctuations in density and composition of the glass waveguide. For example, the fiber cable structure will have imperfections such as impurities, microscopic air bubbles, moisture, etc. The Rayleigh scattering is approximately constant over a given length of a particular optical fiber. As an optical pulse propagates direction along the fiber, a portion of the light which is being scattered is captured by the waveguide structure of the fiber and propagated in the reverse direction, i.e. back to the OTDR. The power level of the back-scattered signal decreases along the length of the fiber because the level of transmitted pulse and the back-scattered light is attenuated over greater lengths of the fiber. The backscattered light typically has a very low level which is inversely proportional to the wavelength of the light raised to the fourth power.
The pulses caused by discrete reflections occur along the fiber length as a result of fiber imperfections, in-line connectors, breaks, or the Fresnel reflection of the end of the fiber. Fresnel reflection also occurs at the connectors because of the glass-air-glass dielectric interface. The level of this reflected light energy is at a relatively high level (relative to the backscattered light) and can be up to four percent of the incident light beam. The Fresnel reflected light level does not change nearly as much as a function of frequency and mode as the level for the Rayleigh backscatter. Thus, to observe the Rayleigh backscatter level and the Fresnel reflection requires an amplifier having a large dynamic range and sensitivity.
The optical time domain reflectometer has become a valuable diagnostic tool for aiding in the detection and measurement of optical features along the length of the fiber. The OTDR generates a fiber response trace by injecting a light pulse into the fiber and detecting and analyzing the characteristics of the backscattered light (i.e. Rayleigh). Once the response trace is generated and displayed, the trace is evaluated for features and optical measurements are made by the operator. In known systems, identification of features and making of measurements are done by a human operator. It will be appreciated that the quality of the OTDR testing and analysis depends largely on the skill level of a human operator.
Therefore, there is a need for a method for automatically finding optical features and making measurements. Not only will this provide improved accuracy, it will also relieve the need for human labour in the optical time reflectometry process.